The present disclosure is directed to a dual antenna system which is useful in locating a drill bit assembly while drilling substantially horizontally under rivers, roads and in other circumstances where the well is substantially horizontal. Horizontal drilling is often used to cross under areas where trenching to bury a pipeline is forbidden. For instance, it may be necessary to cross under a river. Another situation is crossing under a large interstate highway with service roads which might be 250 feet in width. Another example is crossing under an airport runway. In other instances, it may be necessary to drill horizontally to cross under a housing development and the like. In situations of this sort, it is necessary to drill substantially horizontally and yet to know where the drill bit is located during the process of drilling so that the drilled well is formed within a confined region. Typically a designated right of way is furnished for this.
Consider a relatively simple case in which a pipeline is directed at right angles to a river. Assume that the river and the adjacent bank areas are 300 feet in width, and has a water depth of 25 feet. An alternate situation will involve the above mentioned interstate highway. The present apparatus is a system which enables the pipeline to be directed across that area subject to control so that it does not deviate or wander to the right or left. For instance, assume the right of way (ROW hereafter) is 50 feet wide, the drilling rig is situated at one edge of the river or the highway and drilling is initiated from that location. The present apparatus enables the drilling to be carried out so that the drilled hole is in the ROW, and is located at the desired depth. The depths typically range just below the surface. For instance, in passing under a large highway it may be necessary to proceed at a depth of only about 10 feet. Drilling continues until the drill bit is directed back to the surface at the far side. This completes the transhighway tunneling job which can then be interconnected with the remainder of the pipeline, typically constructed by trenching techniques.
The present apparatus enables the crossing to be carried out in a fashion which avoids the difficulties with trenching across the highway or under water, etc. There are multiple techniques available for carrying out such a process. For example, the Goldak firm is the owner of several patents including U.S. Pat. Nos. 3,718,930, also 3,746,106, and 3,975,735. They show structures which are intended to deal with this problem but which are different in operation. There are several patents issued to Coyne which include U.S. Pat. Nos. 3,529,682, also 3,589,454 and also 3,712,391. They all use a certain type of antenna system more specifically set forth. Recently issued U.S. Pat. No. 4,875,014 shows a system using a closed loop antenna laid on the ground which conducts a very substantial current flow. The loop forms a field which is sensed underground. U.S. Pat. No. 4,881,083 uses two antennas which are arranged at a right angle. By contrast with all the foregoing, the present system utilizes a pair of looped antennas which are arranged parallel to the intended pathway of the pipeline. For instance, in passing under a large highway, the present apparatus utilizes two antennas which have the form of thin strips with two or more conductors in each strip. The two antennas are preferably identical. Each antenna is formed of one or more loops, typically a whole number integer where N=2 up to about 100. Each loop antenna has a length which is sufficient to extend beyond where the drill bit is located. For instance, the two loops can each be 100 feet in length, and yet only 1 inch in width, formed of planar material and thereby able to lay flat on the ground. This permits their use over a highway and the like. The two loops are used to receive transmitted signals. However, while two are used, they serve only one directional aspect. They are used to determine centering of the transmitter between the two loops, and that is obtained by a ratio measurement between the two antenna signals. The antennas are preferably constructed with duplicate turns and length. In the system, a drill bit is attached to a motor and is rotated. Immediately adjacent to the motor, the equipment includes a dip sensor arranged to describe the dip orientation of the drill bit. The present apparatus relies on the dip sensors deployed in space to measure the drill bit angle. The angular measurement is coupled with added information regarding the length of drill pipe from the drilling rig to the transmitter, and that enables determination of the location of the drill bit.
The present apparatus is thus summarized as a drilling system which includes a dip sensor located at the drill bit which connects to a transmitter which transmits the measurements from the sensor. In the preferred embodiment, the length of drill pipe extending from the drilling rig is measured. The sensor provides dip angle which help locate in space the end of the column of drill pipe so that its location is known. A dual antenna system is included where two loop antennas are placed lengthwise along the right of way. While they are loop antennas, they are sufficiently narrow that they have an infinitely thin construction. Both receive the transmitted signal. However, they are not used for locating the drill bit depth or direction of drilling. They are used to provide left and right movement of the drill bit.